The invention relates to braking systems for vehicles, provided with a three-way valve simulator and with a brake servo.
For example, document FR-2 895 958 discloses such a system which comprises: a control device serving to receive a command sent by the driver from the brake pedal; a simulator for sending into the pedal a sensation adapted to the driver; a brake servo controlled by a three-way valve of the simulator; and a master cylinder whose actuation takes place with the assistance of the brake servo.
The brake servo comprises a vacuum chamber permanently connected to a vacuum source and a working chamber connected by the three-way valve sometimes to the vacuum chamber and sometimes to an air source at atmospheric pressure. The control of the three-way valve makes it possible, as a function of the pressure prevailing in the working chamber, to move the membrane of the brake servo in order to actuate the master cylinder.
However, the regulations provide that such a system allows the vehicle to brake in the event of failure of the vacuum source. In the aforementioned document, and in such a circumstance, the driver can actuate the pedal so that a rod of the control device, after compensating for an uncoupling play, directly contacts an actuating piston of the master cylinder. Thus, the driver can directly actuate this piston in order to brake the vehicle.
However, in such an operating mode, the simulator continues to provide a sensation to the pedal. Specifically, the actuation of the control device again causes the movement of a control piston of the simulator in the direction of a reaction piston and the compression of a simulation spring interposed between the two in order to oppose them from coming together. When the driver actuates the control rod in order to bring it into contact with the master cylinder piston while compensating for the uncoupling play, he must do so against the pressure of the spring. Therefore, he must provide a specific force for this purpose which consumes some of the driver's energy available for braking. Now, this energy is all the more precious since the braking system, by hypothesis, no longer provides assistance to the driver and since it is the driver himself who must therefore make the pressure in the braking circuit rise. This is therefore a disadvantage insofar as it is preferably sought to preserve under all circumstances the energy that the driver can mobilize to accomplish braking.